The polyethylene naphthalate is commonly produced by the process comprising reacting naphthalenedicarboxylic acid or an ester derivative thereof with ethylene glycol or an ester derivative thereof for esterification to thereby produce an ester of naphthalenedicarboxylic acid and ethylene glycol and carrying out a polycondensation of the above ester of naphthalenedicarboxylic acid and ethylene glycol in the presence of a polycondensation catalyst. In this process for producing the polyethylene naphthalate, a slurry containing naphthalenedicarboxylic acid and ethylene glycol (NDA/EG slurry) is continuously fed to the esterification step. This NDA/EG slurry is required to have excellent fluidity from the viewpoint of, for example, easiness in operation.
The particle sizes of commercially available naphthalenedicarboxylic acid are generally tens of microns, and the particle sizes are so uniform that the fluidity of the NDA/EG slurry is not always good. There is a method of improving the fluidity of the NDA/EG slurry, which comprises regulating the particle sizes of naphthalenedicarboxylic acid. However, the naphthalenedicarboxylic acid exhibits low solubility in most solvents, so that regulating the particle sizes of naphthalenedicarboxylic acid is difficult.
With respect to the method of regulating the particle sizes of naphthalenedicarboxylic acid, for example, Published Japanese Translation of PCT Patent Applications from Other States, No. 505512/1994 describes a process for producing naphthalene-2,6-dicarboxylic acid, which comprises the steps of:
hydrolyzing a dialkyl naphthalene-2,6-dicarboxylate by water at a reaction temperature of at least about 450.degree. F. (232.degree. C.) under liquid phase condition for a period of time sufficient for converting most of the dialkyl naphthalene-2,6-dicarboxylate to naphthalene-2,6-dicarboxylic acid to thereby obtain a reaction product mixture, the above water being present in an amount sufficient for dissolving at least about 10% by weight of formed naphthalene-2,6-dicarboxylic acid at the above reaction temperature (step a); and
recovering the naphthalene-2,6-dicarboxylic acid from the reaction product mixture (step b). In this process, as described in Example 10, the Brookfield viscosity is 800 cP when the molar ratio of EG/NDA is 3, and the Brookfield viscosity is 2430 cP when the molar ratio of EG/NDA is 2.5. Thus, the molar ratio of EG/NDA exhibited when the Brookfield viscosity is 1000 is about 2.9. The naphthalenedicarboxylic acid exhibiting a molar ratio of EG/NDA of about 3 when the Brookfield viscosity is 1000 cP is known as the naphthalenedicarboxylic acid with the highest fluidity.
Of course, when increasing the ratio of ethylene glycol in the NDA/EG slurry, the fluidity of the NDA/EG slurry is improved. However, the increase of the ratio of ethylene glycol is disadvantageous not only from the economic point of view but also in that the probability of formation of reaction products which deteriorate the quality of polyethylene naphthalate is increased.
In the above circumstances, there is a demand for the development of naphthalenedicarboxylic acid particles which can produce an NDA/EG slurry having a low viscosity and the development of a process for producing such naphthalenedicarboxylic acid particles.
The present invention has been made taking the above current situation into account. The object of the present invention is to provide naphthalenedicarboxylic acid particles which exhibit excellent slurry fluidity when formed together with ethylene glycol into a slurry.